Titanium is an example of one of the metals which has found widespread use as an electrically conductive layer in semiconductor microelectronic circuits. Titanium is used in microelectronic devices to provide low-resistance electrical contact to materials, such as silicon or aluminum, that form a stable insulating oxide layer on their surface. Titanium nitride serves as a diffusion barrier to separate silicon from metals in semiconductor devices. Both titanium and titanium nitride serve as bonding agents between materials, such as silicon dioxide and tungsten, which otherwise do not bond strongly together.
This invention relates in particular to a chemical vapor deposition process for depositing titanium and titanium nitride films in a manner that is highly suitable to their application as contact, diffusion barrier and adhesion layers in semiconductor microcircuits.
As microelectronics manufacturers have attempted to make devices that operate faster and are less expensive, they have made narrower openings through which the metallic connections must pass through the insulating layers. The "aspect ratio" is defined as the ratio of the thickness of the insulator layer to the diameter of a hole or the width of a trench through the insulator. In current practice, aspect ratios of 1 or 2 are commonly used. In the next generation of devices, it is widely believed in the industry that aspect ratios of 3 or 4 or more will be used.
Sputtering is usually used to form the titanium and titanium nitride layers in computer processors, memories and other microcircuits. Although sputtering has been successful in coating devices with currently used aspect ratios, it is difficult for sputtering to coat uniformly devices with higher aspect ratios. Sputtering forms coatings which are thicker at the top surface and thinner on the bottoms and lower sidewalls of holes and trenches, and therefore it is said that sputtering has poor "step coverage". While this difficulty of sputtering can be alleviated somewhat by collimating the sputtered material, this leads to other difficulties, including poor sidewall coverage and high cost because of reduced coating rate and greater maintenance requirements.
An alternative coating process with good step coverage would thus be highly useful in semiconductor manufacture. Chemical vapor deposition processes sometimes show very good step coverage, and for this reason CVD processes for titanium and titanium nitride having good step coverage, operating at low enough temperatures, and having relatively non-corrosive byproducts would be advantageous in the manufacture of semiconductor microcircuits.
The requirement for low temperatures is particularly important. In modern semiconductor designs several layers of metal interconnections are applied, and titanium and titanium nitride layers are often used as diffusion barriers and adhesion layers between these successive metal layers. Temperatures during the formation of these upper layers of metallization must be kept below about 400.degree. C. in order to avoid thermal degradation of these structures. Unfortunately, there are no prior art CVD processes for depositing titanium and titanium nitride that meet all of these requirements.
There have been a number of attempts to form titanium by chemical vapor deposition from a number of different reactants. The reaction of titanium tetrahalides with molecular hydrogen is spontaneous only at very high temperatures, which would cause degradation of silicon semiconductor structures. Another difficulty with using this process for semiconductors is that some halogen is deposited as an impurity in the titanium. This residual halogen may cause corrosion of the metal layers. It may also be corrosive to the apparatus used, so that expensive materials of construction must be used.
A lower temperature CVD process for titanium it disclosed in German Patent 1,117,964 (Nov. 23, 1961). This process involves the thermal decomposition of vapors of dicylopentadienyl titanium at temperatures of 260.degree. to 482.2.degree. C. A similar process for depositing titanium, zirconium or hafnium is disclosed in European patent publication 0 468 395 A1 (Jul. 22, 1991) using compounds such as cycloheptatrienyl cyclopentadienyl titanium. Unfortunately, recent attempts to replicate these results have produced high-resistance, porous films containing much more carbon than titanium.
Pure titanium metal has been produced by low-pressure (less than about 0.5 Torr) argon-hydrogen plasma activation of titanium tetrabromide or titanium tetraiodide vapor, as reported by E. T. Eisenbraun, C. Faltermeier, K. Vydianathan, G. Peterson, C. Goldberg, S. Komarov, M. Jones, B. Arkles, A. F. Hepp and E. E. Kaloyeros, paper K5.7 at the fall, 1994 meeting of the Materials Research Society in Boston, Mass. Commercial use of this process is, however, hindered by the difficulty of obtaining reproducible vapor pressures from these solid materials, titanium tetrabromide and titanium tetraiodide. It is also inconvenient to load these hygroscopic solids into bubblers which are conventionally used to supply vapors to such CVD processes.
Titanium nitride has been made by a variety of chemical vapor deposition methods. The earliest CVD process for TiN involves the reaction of titanium tetrachloride vapor with nitrogen and hydrogen at very high temperatures, typically 900.degree. to 1000.degree. C., as reported by W. Schintlmeister, O. Pacher and K. Pfaffinger in the Journal of the Electrochemical Society, volume 123, page 924 in 1976.
The deposition temperature was lowered to temperatures around 400.degree. C. by using preheated ammonia instead of nitrogen and hydrogen, in the process disclosed by Gordon, U.S. Pat. No. 4,535,000 in 1985. Titanium tetrabromide and titanium tetraiodide are slightly more reactive than titanium tetrachloride, particularly at temperatures around 400.degree. C. However, titanium tetrachloride was preferred in this Gordon patent over titanium tetrabromide or titanium tetraiodide, because the latter two compounds are solids which are less convenient to vaporize than is the liquid titanium tetrachloride. Titanium nitride was also deposited by the reaction of titanium tetrabromide and ammonia at substrate temperatures of 500.degree. to 750.degree. C. by Toshiaki Hasegawa et al., Japanese Patent Appln. No. 02-10092 filed Jan. 19, 1990.
Chemical vapor deposition of titanium nitride at temperatures below 400.degree. C. was first achieved by Gordon, Fix and Hoffman and disclosed in U.S. Pat. No. 5,139,825 in 1992. However, the step coverage shown by this process may not be as complete as is desired by the semiconductor industry.